A variety of radio communication systems have evolved having characteristics which, to a large extent, depend upon the propagation characteristics of the radio signals. Low-frequency (LF) and high-frequency (HF) radio signals propagate from one point to another by reflection from the ionosphere. As a result, LF and HF communication systems are capable of communicating messages over long distances. In contrast, very high-frequency (VHF), ultrahigh-frequency (UHF), microwave and higher frequency signals propagate essentially along line-of-sight paths. Thus, VHF, UHF and microwave communications over long distances are generally not possible without utilizing either one or more repeater stations located within sight of each other and the transmitting and receiving stations or by using satellites for relaying the signals. All of these aforementioned systems have limitations which limit their use in specific applications. For example, the use of satellite repeater stations is expensive and often impractical due to the limited number of such stations. Furthermore, the potential unavailability of repeater stations in the event of international hostilities makes VHF, UHF, microwave and higher frequency systems impractical for emergency message communications. While LF and HF systems do not require repeater stations for long-distance communication, the rate at which data can be transmitted using such systems is limited, and such systems are susceptible to interference from other stations. Also, atmospheric disturbances can have unfavorable effects on the ability of the radio waves to reflect from the ionosphere.
The disadvantages of conventional communication systems, some of which are discussed above, has led to the development of communication systems which utilize ionized electron trails created by meteors entering the atmosphere to reflect radio signals in the low VHF range. The meteors generally enter the atmosphere at a height of about 60 miles above the earth's surface, thus allowing long-range communication between stations at distances up to 1200 miles. These trails, called "bursts," are random, but predictable in number. In fact, billions of meteors large enough to give usable trails enter the atmosphere each day.
The typical meteor trail has a useful duration of from a few milliseconds to several seconds. During this time, information can be exchanged between two or more stations. Wait times between suitably located meteor trails can range from a few seconds to minutes, depending upon time of day, time of year, and system design factors. Hence, the transmission between stations consists of "bursts" of high data rate transmissions of tens to hundreds of characters, separated by relatively long periods of silence. One important by-product of the burst characteristic is the ability of many links to share a common transmission frequency, a feature important in data collection systems.
The exchange of information can be in either direction. It can consist of short messages, such as sensor data readout, coded messages of up to several hundred characters, test messages of a few words, or long messages achieved by splicing together the transmissions of successive bursts.
Although meteor communication systems can be used solely for communicating between two stations, it is most commonly used to communicate between a master station and a large number of spaced-apart remote stations. In standard radio communications between master and remote stations, whether or not meteor trails are used for propagation, the naming of the station being called is referred to as "polling." Where all remote stations hear the master station's poll all the time, each remote station replies as it is polled. This protocol prevents simultaneous transmissions to the master station. Two alternative protocols available for one-way or "simplex" communication systems work on exactly opposite principles. The first, time slicing, assigns a time slot to each remote station, which must transmit only during its time slot. The second technique, known as "popcorn," allows each remote station to transmit randomly. Each transmission is repeated several times on the theory that if two remotes transmit simultaneously, they will not transmit simultaneously on the second or third transmission. This technique requires that the total network data transmission time be small relative to the transmit time available.
Selection of an appropriate communications protocol for a meteor burst system is complicated by the intermittent and very short duration nature of the communications path between stations. Simplex systems are generally ruled out since both time slicing and popcorn protocols require the communication link to be available at known times. Therefore, the most common technique uses a process called "probing," where a master station continuously transmits a signal and remote stations continuously listen for the master station's signal. When a meteor trail occurs at the proper location, a remote detects the master's signal, thereby noting the existence of a usable communication link. The remote then transmits its data back to the master station utilizing the same meteor trail from which the probe was reflected.
For small meteor burst networks containing one master station and only a few remotes, the randomness of meteor trails provides a natural control protocol which allows each remote station to transmit whenever it receives a transmission from the master station. The result at the master station is a "popcorn" system, with transmissions from each remote being received randomly.
The natural control protocol inherent in a system employing even a few remotes does not work, however, under certain circumstances. For example, when remote stations are relatively close to the master station, a direct communication link can exist without the need for meteor trails. Under these circumstances, all remote stations in the direct link to the master station will respond to a probe simultaneously. Also, during sporadic electromagnetic radio conditions, large numbers of distant remote stations may receive transmissions from the master station simultaneously for long periods of time. Finally, when remote stations are located in close proximity to each other, they will all simultaneously receive transmissions from the master station reflected from the same meteor trail.
To overcome these and other problems, earlier meteor burst communication systems transmitted a probe containing an address unique to each remote station. Remote stations would thus be required to both receive the transmission from the master station and find its address in the probe before it transmitted a reply. The master station could thus select a remote station from which it wished to receive information until it responded with the appropriate data. This later technique is inherently inefficient because it may take a considerable period of time for a meteor trail to occur at the proper location to establish a communication link with a specific remote station. During this time, in which no information is being conveyed to or from the master station, other meteor trails exist which could be used to communicate with other remote stations. Instead of utilizing these other meteor trails, the master station continues to wait for a communication link to be established with a specific remote station.
The first meteor burst data acquisition system utilizing a large number of remote stations was the United States Department of Agriculture's Soil Conservation Service's snow pack telemetry "SNOTEL" system. The SNOTEL system contains over 500 remote stations. The remote stations measure various weather-related conditions over a wide area in the western states and transmit such information to a master station. The large number of remote stations used in this system made existing communication protocols too restrictive. For this reason, a dynamic variable group addressing structure was developed for the SNOTEL system in which several remote stations are assigned to one or more groups, with all stations in each group responding to the same group address. The number of remote stations in each group being simultaneously addressed varied with the number of bits in the address. Thus, a poll containing a larger number of address bits is more specific to a fewer number of remote stations. An address containing a few number of bits is more general and allows a larger number of remote stations to respond to the poll. For example, an address of "225" may allow only remote station no. 225 to respond to a poll. An address of "22" would allow remote stations nos. 220-229 to respond to the probe. Finally, an address of "2" would allow remote station nos. 200-299 to respond to the probe. This technique, known as "variable length addressing," is described and claimed in U.S. Pat. No. 4,277,845, issued to Smith et al.
Although the advantages of the variable length addressing technique allowed implementation of a system having a large number of remote stations, it nevertheless exhibited certain shortcomings. Perhaps the biggest problem with variable length addressing is the vulnerability of the remote stations to a master station failure. Specifically, a remote station has a limit to how many times it can transmit data in a given period of time without exhausting its battery. This limit is determined by the energy expended per transmission and the charge rate of the battery which, in the SNOTEL system, is accomplished by a solar charger. Master station failures sometimes allow the master station to transmit its poll but not detect replies from remote stations. As a result, the master station continues to attempt communication with remote stations in the selected group, and the remote stations in that group continue to reply to the probe without the master station acknowledging the reply and polling another group of remote stations. Another problem with the variable address technique results from the physical separation of remote stations in the same group. Remote stations in close proximity must be in different groups so that they will not simultaneously respond to the same meteor trail. Thus, addressing for the entire network of remote stations must be generated before the first remote can be deployed. Once a remote station is deployed, it can be reassigned to another group only by visiting the site. Finally, variable length addressing has not been totally effective in controlling spurious remote station replies. During noise and weak signal conditions, the signal received by a remote station from a master station contains too many errored bits, allowing it to detect the address of its group when, in fact, that address was not transmitted. This problem is magnified by the nature of variable length addressing because a larger number of remote stations are capable of responding to a probe as the number of bits in the probe's address is reduced. Thus, an errored bit in a short address is likely to affect a large number of remote stations. This results in a multitude of unsolicited remote station replies which interfere with replies from the intended remote station and which can excessively discharge the remote station's battery.